Purpose of cellular metabolism

Most common form of energy

To make energy, cells require 2 things

Obtaining nutrients from diet

Cells break down organic molecules to obtain energy, usually in the form of ATP. Chemical reactions within mitochondria provide most of the energy needed by a typical cell for its varied activities.
To carry out their energy-generating processes, cells in the human body must also obtain oxygen & nutrients.
Whereas oxygen is absorbed at the lungs, nutrients - essential substances such as water, vitamins, ions, carbohydrates, lipids, & proteins - are obtained from the diet by absorption at the digestive tract.

How are nutrients & oxygen delivered to cells throughout the body?

relationship between catabolism & metabolism in nutrition

carbohydrate

A carbohydrate is an organic molecule that contains carbon, hydrogen, & oxygen in a ratio near 1:2:1.
Familiar carbohydrates include the sugars & starches that make up roughly half of the typical U.S. diet.

Carbohydrates are most important as sources of energy.
Name the 3 major types of carbohydrates:

monosaccharides

A simple sugar, or monosaccharide is a carbohydrate containing from three to seven carbon atoms.
Included within this group is glucose, the most important metabolic "fuel" in the body. Glucose & other monosaccharides dissolve readily in water.

disaccharide

Two monosaccharides joined together form a disaccharide.
Disaccharides such as sucrose (table sugar) have a sweet taste &, like monosaccharides, are soluble in water.
Many foods contain disaccharides, but all carbohydrates except monosaccharides must be disassembled through hydrolysis before they can provide useful energy.
Most sweet junk foods, such as candy & soft drinks, abound in simple sugars (commonly fructose) & disaccharides (generally sucrose).

polysaccharide

Larger carbohydrate molecules are called polysaccharides.
They result when repeated dehydration synthesis reactions add additional monosaccharides or disaccharides.
Starches are glucose-based polysaccharides important in our diets.
Most starches are manufactured by plants. Your digestive tract can break these molecules into simple sugars.
Starches found in potatoes & grains are important energy sources.
In contrast, cellulose, a component of the cell walls of plants, is a polysaccharide that our bodies cannot digest. The cellulose of foods such as celery contributes to the bulk of digestive wastes but is useless as an energy source.

hydrolysis of disaccharides & some polysaccharides

Dehydration synthesis, or condensation, links molecules together by the removal of a water molecule.
The breakdown of sucrose into simple sugars is an example of hydrolysis, the functional opposite of dehydration synthesis

glycogen

Glycogen, or animal starch, is a polysaccharide composed of interconnected glucose molecules.
Like most other large polysaccharides, glycogen will not dissolve in water or other body fluids.
Liver & muscle tissues make & store glycogen. When these tissues have a high demand for energy, glycogen molecules are broken down into glucose; when demands are low, the tissues absorb glucose from the bloodstream & rebuild glycogen reserves.

4 basic reasons cells synthesize new organic components:

1. To perform structural maintenance & repairs (all cells must expend energy for ongoing maintenance & repairs because most structures in the cell are temporary, not permanent. The continuous removal & replacement of these structures are part of the process of metabolic turnover);
2. To support growth (cells preparing to divide enlarge & synthesize extra proteins & organelles);
3. To produce secretions (secretory cells must synthesize their products & deliver them to the interstitial fluid);
4. To build nutrient reserves (most cells "prepare for a rainy day" - some emergency, an interval of extreme activity, or a time when the nutrient supply in the bloodstream is inadequate - by storing nutrients in a form that can be mobilized as needed. For example, muscle cells store glucose in the form of glycogen, adipocytes store triglycerides, & liver cells store both).

BMR

Basal Metabolic Rate: The resting metabolic rate of a fasting subject under normal homeostatic conditions.
The result can be measured in terms of calories per hour, per day, or per unit of body weight per day.

Lipids

Proteins

Proteins are the most abundant organic components of the human body.
All proteins contain carbon, hydrogen, oxygen, & nitrogen; smaller quantities of sulfur may also be present.

Proteins are long chains of organic molecules called amino acids.

The human body contains significant quantities of the 20 different amino acids that are the building blocks of proteins.

A typical protein contains 1000 amino acids, but the largest protein complexes may have 100,000 or more.

The individual amino acids are strung together like beads on a string, with the carboxylic acid group of one amino acid attached to the amino group of another. This connection is called a peptide bond.

Peptides are molecules made up of amino acids held together by peptide bonds. If a molecule consists of two amino acids, it is called a dipeptide. Polypeptides are long chains of amino acids. Polypeptides containing more than 100 amino acids are usually called proteins.

Cellular Metabolism

Cells obtain organic molecules from the extracellular fluid & break them down to obtain ATP. Only about 40% of the energy released through catabolism is captured in ATP; the rest is radiated as heat.
The ATP generated by catabolism provides energy for all vital cellular activities, including anabolism.

3 sources of organic molecular energy & what they are broken down into:

Percenage of energy captured vs released as heat:
40% captured as ATP
60% released as heat

As mitochondrial enzymes break the covalent bonds that hold these molecules together, they capture roughly 40% of the energy released. The captured energy is used to convert ADP to ATP, & the rest escapes as heat that warms the interior of the cell & the surrounding tissues.

TCA Cycle & Electron Transport System

Mitochondria absorb small carbon chains produced by the breakdown of fatty acids, glucose, & amino acids from the nutrient pool. The small carbon chains are broken down further by means of the tricarboxylic acid (TCA) cycle & the electron transport system.

Carbs & Cellular Metabolism

Carbohydrates, most familiar to us as sugars and starches, are important sources of energy. Most cells generate ATP and other high-energy compounds by breaking down carbohydrates, especially glucose. The complete reaction sequence can be summarized as:

GLYCOLYSIS OCCURS WHERE?

Aerobic vs Anaerobic

Because the steps in the cytosol do not require oxygen, they are said to be anaerobic.
The subsequent reactions, which occur within mitochondria, consume oxygen & are thus aerobic.
The mitochondrial activity responsible for ATP production is called aerobic metabolism, or cellular respiration.

Net gain of ATP during Glycolysis:

The reaction sequence of glycolysis yields a net gain of two ATP molecules for each glucose molecule converted to two pyruvic acid molecules.

A few highly specialized cells, such as red blood cells, lack mitochondria & derive all of their ATP by glycolysis.
Skeletal muscle fibers rely on glycolysis for energy production during periods of active contraction, and most cells can survive brief periods of hypoxia (low oxygen levels) by using the ATP provided by glycolysis alone.
When oxygen is readily available, however, mitochondrial activity provides most of the ATP required by body cells.

KNOW THE AEROBIC VS ANAEROBIC YIELD OF ATP:

Within a cell's cytoplasm, glycolysis breaks down a six-carbon glucose molecule into two three-carbon pyruvic acid molecules.
This process involves a series of enzymatic steps.
A net gain of two ATPs results for each glucose molecule converted to pyruvic acid.

Points to Emphasize: Glycolysis yields a mere two ATP molecules/glucose molecule, all that can be gained without oxygen.

Aerobic metabolism liberates 17 times more ATP than anaerobic!

These chemical facts dramatize the crucial importance of oxygen to human life.

Pyruvic Acid

Even though glycolysis yields an immediate net gain of two ATP molecules for the cell, a great deal of additional energy is still stored in the chemical bonds of pyruvic acid.
The cell's ability to capture that energy depends on the availability of oxygen.
If oxygen supplies are adequate, mitochondria will absorb the pyruvic acid molecules & break them down completely.
The hydrogen atoms of pyruvic acid are removed by coenzymes & are ultimately the source of most of the cell's energy gain.
The carbon & oxygen atoms are removed & released as carbon dioxide.

The Electron Transport System & ATP Formation

The electrons of hydrogen atoms from the TCA cycle are transferred by coenzyme Q to the ETS (a series of cytochrome molecules), & the hydrogen ions (H+) remain in the matrix.
The energy carrying electrons are passed from one cytochrome to another.
Energy released by the passed electrons is used to pump H+ from the matrix into the intermembrane space.
This creates a difference in the concentration of H+ across the inner membrane.
The hydrogen ions then diffuse through ATP synthase in the inner membrane, & their kinetic energy is used to generate ATP.

The electron transport system (ETS) is embedded in the inner mitochondrial membrane.

The ETS consists of an electron transport chain made up of a series of protein-pigment complexes called cytochromes.

The ETS does not produce ATP directly. Instead, it creates the conditions necessary for ATP production.

The hydrogen atoms from the TCA cycle do not enter the ETS intact. Only the electrons (which carry the energy) enter the ETS; the protons that accompany them are released into the mitochondrial matrix.

The electrons from both paths are passed from coenzyme Q to the first cytochrome & then from cytochrome to cytochrome, losing energy in a series of small steps. At several steps along the way, this energy is used to drive hydrogen ion pumps that move hydrogen ions from the mitochondrial matrix into the intermembrane space between the two mitochondrial membranes.

This creates a large concentration gradient of hydrogen ions across the inner membrane, so the hydrogen ions then diffuse back into the matrix through a membrane enzyme called ATP synthase. The kinetic energy of the passing hydrogen ions is used to attach a phosphate group to ADP, forming ATP. This process is called chemiosmosis, a term that links the chemical formation of ATP with transport across a membrane. At the end of the electron transport system, an oxygen atom accepts the electrons & combines with two hydrogen ions to form a molecule of water.

Importance of the ETS

The electron transport system is the most important mechanism for the generation of ATP; in fact, it provides roughly 95% of the ATP needed to keep our cells alive.

Halting or significantly slowing the rate of mitochondrial activity will usually kill a cell. If many cells are affected, the individual may die. If, for example, the cell's supply of oxygen is cut off, mitochondrial ATP production will cease because the ETS will be unable to pass along its electrons.

ETS and ATP yield

KEY POINT:
For each glucose molecule processed, a typical cell gains 36 molecules of ATP.
All but two of them are produced within mitochondria.

1. During glycolysis in the cytoplasm, the cell gains two molecules of ATP for each glucose molecule broken down to pyruvic acid.

2. Inside the mitochondria, the two pyruvic acid molecules derived from each glucose molecule are fully broken down in the TCA cycle. Two revolutions of the TCA cycle, each yielding a molecule of ATP, provide a net gain of two additional molecules of ATP.

3. For each molecule of glucose broken down, activity at the electron transport chain in the inner mitochondrial membrane provides 32 molecules of ATP.

Alternative Catabolic Pathways:

2 ATP is always...?

Lipid Metabolism

A cell generates 144 ATP molecules from the breakdown of one 18-carbon fatty acid molecule—almost 1.5 times the energy obtained from the breakdown of three six-carbon glucose molecules.

Because they are insoluble in water, lipids are stored in compact droplets in the cytosol. However, if the droplets are large, it is difficult for water-soluble enzymes to get at them. This makes lipid reserves more difficult to access than carbohydrate reserves.

chylomicrons

Lipoproteins are classified by size and by their relative proportions of lipid and protein. One group, the chylomicrons, forms in the intestinal tract. Chylomicrons are the largest lipoproteins, and some 95% of their weight consists of triglycerides. Chylomicrons transport triglycerides absorbed from the intestinal tract to the bloodstream, from which they are absorbed by skeletal muscle, cardiac muscle, adipose tissue, and the liver.

Two other major groups of lipoproteins are the low-density lipoproteins (LDLs) and high-density lipoproteins (HDLs). These lipoproteins are formed in the liver and contain few triglycerides. Their main roles are to shuttle cholesterol between the liver and other tissues. LDLs deliver cholesterol to peripheral tissues. Because LDL cholesterol may end up in arterial plaques, it is often called "bad cholesterol." HDL cholesterol transports excess cholesterol from peripheral tissues to the liver for storage or excretion in the bile. Because HDL cholesterol does not cause circulatory problems, it is called "good cholesterol."

Protein Metabolism

Transamination attaches the amino group of an amino acid to another carbon chain, creating a "new" amino acid. Transaminations enable a cell to synthesize many of the amino acids needed for protein synthesis.

deamination is the removal of an amino group in a reaction that generates an ammonia molecule (NH3).

Protein Metabolism

Peptide bonds are broken, and the free amino acids are used to manufacture new proteins. If other energy sources are inadequate, mitochondria can break down amino acids in the TCA cycle to generate ATP.

Sources of Amino Acids

Of the 20 amino acids needed to create proteins, 10 must be obtained from the diet because human metabolism can't supply them. They are termed the essential amino acids because even if only one is lacking, protein synthesis ceases. Dietary shortages of amino acids cause grave illness and death. Each year, more than 5 million children under the age of 5 die worldwide from protein-energy malnourishment.

TYPICAL MIXED DIET IN THE U.S.

Calorie vs calorie
(NOTE LOWER CASE vs UPPER CASE "C")

Inside cells, some of energy may be captured as ATP, but much of it is lost to the environment as heat.
The unit of energy measurement is the calorie (cal),the amount of energy required to raise the temperature of 1 g of water 1° celsius.

One gram of water is not a very practical measure when you are interested in the metabolic operations that keep a 70-kg human alive, however, so the kilocalorie (kcal), or Calorie (Cal), is used instead.

One Calorie is the amount of energy needed to raise the temperature of 1 kilogram of water 1° centigrade.

1. Radiation.
Warm objects lose heat energy as infrared radiation. When we feel the sun's heat, we are experiencing radiant heat. Your body loses heat the same way. More than half of the heat you lose occurs by radiation, (primarily infrared).

2. Conduction.
Conduction is the direct transfer of energy through physical contact.
When you sit on a cold plastic chair in an air-conditioned room, you are immediately aware of this process. Conduction is generally not an effective mechanism of gaining or losing heat, except in a manner opposite of that desired, (i.e., trauma victims lying a ground cooler than 98F (37C) rapidly cool, partly through conduction.)

3. Convection.
Convection is the result of conductive heat loss to the air that overlies the surface of an object.
Warm air rises because it is lighter than cool air. As your body conducts heat to the air next to your skin, that air warms and rises, moving away from your skin surface. Cooler air replaces it, and as this air in turn warms, the pattern repeats.

4. Evaporation.
When water evaporates, it changes from a liquid to a vapor. This process absorbs energy—roughly 580 calories (0.58 Cal) per gram of water evaporated— and, thus, cools any surface on which it occurs.
The rate of evaporation and heat loss occurring at your skin is highly variable. Each hour, 20-25 ml of water crosses epithelia and evaporates from the alveolar surfaces of the lungs and the surface of the skin. This insensible perspiration remains relatively constant; it accounts for roughly one-fifth of the average heat loss from a body at rest. The sweat glands responsible for sensible perspiration have a tremendous scope of activity, ranging from virtual inactivity to secretory rates of 2-4 liters (or 2-4 kg) per hour. This is equivalent to an entire day's resting water loss in under an hour.

Which part of the brain is the "body's thermostat"?

HYPOTHALAMUS

When body is too hot, 'heat-loss center' is activated (parasympathetic);

When body is too cold, 'heat-gain center' is activated (sympathetic).

Heat loss & heat gain requires the coordinated activity of many different systems.
That activity is coordinated by the heat-loss center and heat-gain center of the hypothalamus.
The HEAT-LOSS center adjusts activity through the PARASYMPATHETIC division of the autonomic nervous system,
whereas the HEAT-GAIN center directs its responses through the SYMPATHETIC division.

When the temperature at the heat-loss center exceeds its set point, three responses occur:
1. Dilation of skin blood vessels;
2. Increas of sweat gland secretion;
3. Acceleration of ventilation.

1. Peripheral blood vessels dilate, sending warm blood flowing to the surface of the body. White skin takes on a reddish color and all skin rises in temperature; heat loss through radiation and convection increases.
2. Sweat glands are stimulated, and as perspiration flows across the skin, heat loss through evaporation accelerates.
3. The respiratory centers are stimulated, and the depth of respiration increases. The individual often begins respiring through the mouth, enhancing heat loss through increased evaporation from the lungs.

Functions of 'heat-gain center'

Heat is conserved by decreasing blood flow to the skin, thereby reducing losses by radiation, convection, and conduction.
The skin cools, & with blood flow restricted, it may take on a bluish or pale coloration. In addition, blood returning from the limbs is shunted into a network of deep veins that lies beneath an insulating layer of subcutaneous fat. (Under warm conditions, blood flows through a more superficial venous network, through which heat can be lost.)

In addition to conserving heat, the heat-gain center stimulates two mechanisms that generate heat:

In shivering thermogenesis, muscle tone is gradually increased until stretch receptors stimulate brief, oscillatory contractions of antagonistic skeletal muscles. The resulting shivering stimulates energy consumption by skeletal muscles, & the generated heat warms the deep vessels to which the blood has been diverted. Shivering can increase the rate of heat generation by as much as 400%.

In non-shivering thermogenesis, hormones are released that increase the metabolic activity of cells in all tissues. Epinephrine from the adrenal gland immediately increases the breakdown of glycogen & glycolysis in the liver and in skeletal muscles and increases the metabolic rate in most tissues. The heat-gain center also stimulates the release of thyroxine by the thyroid gland, accelerating carbohydrate use & the breakdown of all other nutrients. These effects develop gradually over a period of days to weeks.